Add info on installing with CRT support (#2812)

* Add info about CRT support
This update adds a new section to the Quickstart article,
"Using the AWS Common Runtime (CRT)". This section covers
(briefly) what CRT is and why it matters, then what it's
currently used for, plus how to install and remove CRT
support from a project.

docs/source/guide/quickstart.rst

@@ -56,6 +56,37 @@ certain versions, you may provide constraints when installing::
 
    The latest development version of Boto3 is on `GitHub <https://github.com/boto/boto3>`_.
 
+Using the AWS Common Runtime (CRT)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In addition to the default install of Boto3, you can choose to include the new `AWS Common Runtime <https://docs.aws.amazon.com/sdkref/latest/guide/common-runtime.html>`_
+(CRT). The AWS CRT is a collection of modular packages that serve as a new foundation for AWS SDKs.
+Each library provides better performance and minimal footprint for the functional area it
+implements. Using the CRT, SDKs can share the same base code when possible, improving consistency
+and throughput optimizations across AWS SDKs.
+
+When the AWS CRT is included, Boto3 uses it to incorporate features not otherwise
+available in the AWS SDK for Python.
+
+At this time, Boto3 uses the AWS CRT's authentication package (`aws-c-auth
+<https://github.com/awslabs/aws-c-auth>`_) to add support for the `AWS Signature Version 4
+<https://docs.aws.amazon.com/general/latest/gr/signature-version-4.html>`_ (sigv4) signer, which
+adds authentication to your AWS requests using your security credentials (your AWS access key and
+secret access key).
+
+Boto3 doesn't use the AWS CRT by default but you can opt into using it by specifying the
+:code:`crt` `extra feature <https://www.python.org/dev/peps/pep-0508/#extras>`_ when installing Boto3::
+
+    pip install boto3[crt]
+
+To revert to the non-CRT version of Boto3, use this command::
+
+    pip uninstall awscrt
+
+If you need to re-enable CRT,  reinstall :code:`boto3[crt]` to ensure you get a compatible version of :code:`awscrt`::
+
+    pip install boto3[crt]
+
 Configuration
 -------------